Haas helped out after Murray injured his ankle

Murray needed treatment on his right foot in the first set and fell heavily on his left ankle early in the second.

But he came back from a lengthy delay to win the next game and open up a 3-0 lead and managed to close out the set.

Murray saved two match points in the deciding tie-break before clinching a superb win to reach the semi-finals.

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"I can't really explain how I won the match," said the 19-year-old Scot.

"My ankle was hurting after the fall but my right hip hurt the most after I landed badly on that and I also banged my knee.

"I just hope that when I wake up that everything is not too sore because the important thing is my health as I've hurt my ankle before and don't want to risk anything."

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Early on, the ninth-seeded Haas looked in control of the match, dropping only four points in his first three service games.

Murray had breaks points for 4-3 but missed out and was made to pay as the German took the set, with a foot injury adding to his discomfort.

The second set began encouragingly for Murray as he claimed his first break of the game, but when he overstretched for a ball, he went down in evident pain and needed a long break for treatment.

But the delay did nothing to blunt his concentration as he took the set comfortably.

After five deuces, Murray looked to have gained a vital advantage when he broke for 4-3 in the deciding set, only for Haas to break straight back.

Then it was the German's turn to need injury treatment before the tie-break.

Haas had two match points but Murray held his nerve to finally wrap up the win after nearly three hours, and set up a semi-final clash against Serbia's Novak Djokovic.

The 12th seed reached the last four with a fairly straightforward 6-3 6-4 win over David Ferrer.

Second seed Rafael Nadal of Spain and third seed Andy Roddick of the United States will contest the other semi-final on Saturday.